The present invention relates to an optical information recording medium capable of reproducing information signals or recording and reproducing information signals by using light beams, a manufacturing method therefor, a manufacturing apparatus therefor and an optical information recording and reproducing apparatus making use thereof, and more particularly to an optical information recording medium having a multilayer structure including a plurality of information layers, a manufacturing method therefor, a manufacturing apparatus therefor and an optical information recording and reproducing apparatus making use thereof.
Hitherto, an optical information recording medium has been known which is capable of optically recording information signals or reproducing recorded information signals and which is formed into an optical disk, an optical card or the like. In general, the foregoing recording medium uses a semiconductor laser unit as a light source. By irradiating the recording medium with light beams finely converged by a lens, information signals can be recorded on the recording medium in a large quantity, and information signals recorded on the recording medium can be reproduced.
At present, investigations have been performed to further enlarge the recording capacity of the recording medium of the foregoing type. To raise the recording density, it is effective to improve the reproducing resolution by finely limiting the light beam. In order to achieve this, investigations have been performed such that the wavelength of the light beam is shortened or the numerical aperture (NA) is enlarged. Moreover, reproducing methods have been investigated in which the focusing or tracking accuracy is improved and crosstalk between signals is prevented so as to allow the recording surface density to be raised.
Although employment of the foregoing methods enables the recording capacity per unit area to be somewhat enlarged, a structure having only a single information layer for recording information encounters a limitation when the recording density is intended to be raised.
If a plurality of information layers for recording information are provided, it can be expected that the recording capacity can be multiplied. A method for manufacturing an optical disk having the multilayer structure has been disclosed in, for example, U.S. Pat. No. 5,126,996.
A process for manufacturing the foregoing optical disk will now be described. As shown in FIG. 21(a), a first information layer 212 is formed on the surface of a substrate 211 manufactured by an injection molding method or the like and having information pits. Then, as shown in FIG. 21(b), a photosetting resin 214 is applied to the upper surface of a master 213 having information pits. Then, as shown in FIG. 21(c), the surface of the first information layer 212 on the substrate 211 having the information pits and the surface of the master 213 having the information pits are caused to face each other. Then, in a state where the substrate 211 is pressed, the photosetting resin 214 is irradiated with light from a position on the outside of the master 213. As a result, the photosetting resin 214 is set so that the photosetting resin 214 is adhesive-bonded to the first information layer 212. Then, as shown in FIG. 21(d), the master 213 is removed from the photosetting resin 214. As a result, a resin layer having information pits on the surface thereof and composed of the photosetting resin 214 can be formed. Then, as shown in FIG. 21(e), a second information layer 215 is formed on the resin layer (made of the photosetting resin 214). Finally, as shown in FIG. 21(f), a protective coating layer 206 is formed on the second information layer 215. As a result of the foregoing process, an optical disk having a double-layer structure can be obtained.
However, when the master 213 is separated from the photosetting resin 214 (see FIG. 21(d)), the foregoing conventional manufacturing method can involves separation being easily taking place at the interface between the first information layer 212 and the substrate 211 or the resin layer (made of the photosetting resin 214). Thus, there arises a problem in that the manufacturing yield has been unsatisfactory. The reason for this can be considered that the adhesivity between the master 213 and the resin layer (made of the photosetting resin 214) is made to be greater than that between the first information layer 212 and the substrate 211 or that between the first information layer 212 and the resin layer (made of the photosetting resin 214).
In a case where the substrate 211 is made of a resin, change in the environmental temperature or in the humidity sometimes raises a problem in that the manufactured optical disk can be deformed or an error occurs in reproducing the signals.
Moreover, an apparatus of a type for reproducing information signals from a plurality of information layers has a problem in that the servo operation becomes instable due to an influence of light reflected by an information layer other than the subject information layer.
The present invention was found to overcome the foregoing problems experienced with the conventional structure, and, therefore, an object of the present invention is to provide an optical information recording medium having a multilayer structure which cannot easily be deformed due to change in the environment, a manufacturing method therefor, a manufacturing apparatus therefor and an apparatus for optically recording and reproducing information.
To achieve the foregoing object, according to one aspect of the present invention, there is provided an optical information recording medium having two different information layers and capable of reproducing information signals from the information layers, recording information signals on the information layers that can reproduce the recorded information signals when irradiated with a light beam, the optical information recording medium comprising: a first substrate having, on one side thereof, one or more selected from the group consisting of information pits corresponding to information signals, guide grooves for tracking and sample pits; a first information layer formed on either surface of the first substrate and having a predetermined transmissivity and a predetermined reflectance with respect to the light beam; a second substrate having, on one side thereof, one or more selected from a group consisting of information pits corresponding to information signals, guide grooves for tracking and sample pits; a second information layer formed on either surface of the second substrate and having a predetermined reflectance; and a separation layer formed between the first information layer and the second information layer that is transparent with respect to the light beam. The structure of the optical information recording medium provides a double-layer recording medium that is capable of reproducing information signals from the first and second information layers, recording information signals on the first and second information layers and reproducing recorded information signals.
It is preferable that the optical information recording medium according to the present invention has a structure such that the thickness of the first substrate is substantially the same as the thickness of the second substrate. As a result of the preferred structure, a structure vertically symmetrical with respect to the separation layer can be formed. Therefore, even if stress or the like is generated in the substrate due to change in the temperature or the like when the optical information recording medium is manufactured, distortion can be compensated. Even if unusual deformation factors act on the two substrates due to change in the temperature or the humidity of the environment, deformation and warp can be inhibited. As a result, a structure durable against change in the environment can be obtained. Therefore, even if the first and second substrates are made of resin, deformation of the manufactured recording medium causing an error in reproducing a signal can be prevented.
It is preferable that in the optical information recording medium according to the present invention, the below mentioned formula is satisfied;   R1  ≈      1    -    A1    +                  (                  2          ·          R2                )                    -        1              -                  {                                            [                              1                -                A1                +                                                      (                                          2                      ·                      R2                                        )                                                        -                    1                                                              ]                        2                    -                                    (                              1                -                A1                            )                        2                          }            0.5      
wherein R1 is a reflectance of the first information layer,
A1 is an absorption ratio of the first information layer, and
R2 is a reflectance of the second information layer with respect to the wavelength of the light beam used to reproduce the information.
It is preferable that the optical information recording medium has a structure such that the reflectance of the first information layer is 25% to 40%. As a result of the foregoing preferred structure, a recording medium having the first and second information layers, which are reproduction only information layers, can be obtained in which the amplitudes of the signals from the first information layer and those from the second information layer are similar to each other and the reproducing amplitude is satisfactorily large.
It is preferable that the optical information recording medium according to the present invention has a structure such that the second information layer includes a reflecting layer, a first dielectric-material layer, a recording layer and a second dielectric-material layer, which are sequentially laminated on the second substrate. According to the foregoing preferred structure, a second information layer, the information of which can be rewritten, can be obtained.
It is preferable that the optical information recording medium according to the present invention has a structure such that the first information layer has a coefficient of absorption which is substantially zero with respect to the light beam. According to the foregoing preferred structure, the quantity of the light that reaches the second information layer can be enlarged.
It is preferable that the optical information recording medium according to the present invention has a structure such that the first information layer includes at least two layers of thin films. According to the foregoing preferred structure, the reflectance of the first information layer can be enlarged and the quantity of the light that reaches the second information layer can be enlarged. In the foregoing case, it is preferable that the first information layer includes a first dielectric-material layer, a recording layer and a second dielectric-material layer, which are sequentially laminated on the first substrate. According to the foregoing preferred structure, a first information layer can be obtained on which information can be recorded or information of which can be rewritten.
It is preferable that the optical information recording medium according to the present invention has a structure such that both of the first and second substrates have information pits corresponding to information signals on either surface. According to the foregoing preferred structure, a recording medium having a large capacity can be obtained.
It is preferable that the optical information recording medium according to the present invention has a structure such that the first information layer is a reproduction only information layer and the second information layer is a recording and reproducing information layer. According to the foregoing preferred structure, light absorption of the reproduction only first information layer can be set to a low level. Moreover, since the influence of diffraction occurring due to information pits can be inhibited, information can be recorded or reproduced with small power.
It is preferable that the optical information recording medium according to the present invention has a structure such that the thickness of the separation layer is larger than the focal depth of an optical system for converging the light beam and smaller than the tolerance of a base allowed by the optical system. According to the foregoing preferred structure, information satisfactorily free from crosstalk can be reproduced from another information layer as long as the aberration of the light beam to be applied is small.
It is preferable that the optical information recording medium according to the present invention has a structure such that information pits or sample pits are formed on the first and second substrates and the shapes of the pits formed on the first substrate are different from the shapes of the pits formed on the second substrate. In the foregoing case, it is preferable that the width of the pit formed on the second substrate is larger than the width of the pit formed on the first substrate. According to the foregoing preferred structure, diffraction of light reflected from the first information layer and that from the second information layer can be made to correspond. Therefore, signals can be reproduced stably.
It is preferable that the optical information recording medium according to the present invention has a structure such that guide grooves are formed in the surfaces of the first and second substrates and the width of the guide groove formed in the second substrate is larger than the width of the guide groove formed in the first substrate. According to the foregoing preferred structure, the effect of the guide grooves formed in the surface of the first substrate and the effect of the guide grooves formed in the surface of the second substrate can be made to be the same.
It is preferable that the optical information recording medium according to the present invention has a structure such that information pits are formed on the surfaces of the first and second substrates and the density of the information pits formed on the second substrate per unit area is lower than the density of the information pits formed on the first substrate per unit area. According to the foregoing preferred structure, information can be reproduced satisfactorily from an information layer out of the focal depth of the optical system for converging the light beam.
It is preferable that the optical information recording medium according to the present invention has a structure such that sample pits or guide grooves are formed in the surfaces of the first and second substrates and the pitch of the sample pits or the guide grooves formed in the second substrate is shorter than the pitch of the sample pits or the guide grooves formed in the first substrate. According to the foregoing preferred structure, a recording medium can be obtained in which signals can be reproduced satisfactorily from an information layer out of the focal depth of the optical system for converging the light beam and signals can be recorded on the same.
It is preferable that the optical information recording medium according to the present invention has a structure such that information pits or sample pits are formed on the surfaces of the first and second substrates and the directions of the information pits are opposite to each other between the first substrate and the second substrate when viewed from a position upon which the light beam is made incident. According to the foregoing preferred structure, the material and molding process of the substrates can be made to be the same as well as the mastering process. As a result, it is only necessary to prepare two types of manufacturing apparatuses having the same function or one manufacturing apparatus can be used commonly when the substrates are manufactured. Thus, the cost of the facilities for manufacturing the substrates can be reduced.
It is preferable that the optical information recording medium according to the present invention has a structure such that the information pits, the guide grooves or the sample pits formed on the surfaces of the first and second substrates are formed into a spiral shape relative to the central portions of the first and second substrate and the shape of the spiral on the first substrate and the shape of the spiral on the second substrate are the same when viewed from a position upon which the light beam is made incident. According to the foregoing preferred structure, the light beam is moved in one direction from the inner portion to the outer portion or one direction from the outer portion to the inner portion regardless of the information layer having the information pits to which tracking has been made. In the case where the structure is employed in which the light beam is moved from the inner portion to the outer portion, a reproduction method may be employed in which administration information is detected in the inner portion of any of the information layers and an access is made to a required information region including the portion between the information layers. Therefore, it can be said that the foregoing structure is suitable for a recording medium capable of allowing a high speed access to the information layers.
It is preferable that a pair of the optical information recording mediums according to the present invention and an adhesive-bonding layer are provided and the second substrates of the pair of optical information recording mediums are adhesive-bonded to each other through the adhesive-bonding layer. According to the foregoing preferred structure, a recording medium having a four-layer structure can be obtained in which information can be recorded and reproduced from each of the information layers by irradiating the two sides with the light beams. In the foregoing case, it is preferable that the thicknesses of the first substrates of the pair of optical information recording mediums are substantially the same and the thicknesses of the second substrates of the pair of optical information recording mediums are substantially the same.
According to another aspect of the present invention, there is provided a method of manufacturing an optical information recording medium having two different information layers and capable of reproducing information signals from the information layers, recording information signals on the information layers and reproducing the recorded information signals when irradiated with a light beam. The method of manufacturing an optical information recording medium comprises: a first film-forming step of forming a first information layer having a predetermined transmissivity and a predetermined reflectance on a substrate having, on one side thereof, one or more selected from the group consisting of information pits corresponding to information signals, guide grooves for tracking and sample pits; a second film-forming step of forming a second information layer having a predetermined reflectance on a second substrate having, on one side thereof, one or more selected from a group consisting of information pits corresponding to information signals, guide grooves for tracking and sample pits; an applying step of applying a resin layer, which is transparent with respect to the light beam, to the upper surface of the first information layer or the second information layer; an adhesive-bonding step of causing the first information layer and the second information layer to face each other and adhesive-bonding the first and second information layers to each other through the resin layer. According to the structure of the method of manufacturing the optical information recording medium, the step of removing the master is not required. By simply adhesive-bonding to each other the substrates having the information pits formed previously, a recording medium having a double-layer structure can be obtained. As a result, the manufacturing yield can be improved.
It is preferable that the method of manufacturing an optical information recording medium according to the present invention has a structure such that the thickness of the first substrate is substantially the same as the thickness of the second substrate.
It is preferable that the method of manufacturing an optical information recording medium according to the present invention further comprises a pressing step of pressing the first and second substrates from the outsides of the first and second substrates. According to the foregoing, a resin layer satisfactorily free from irregularity in thickness can be formed.
It is preferable that in the method of manufacturing an optical information recording medium according to the present invention a photosetting resin is used to form the resin layer and the resin layer is irradiated with light from a position on the outside of the first substrate so as to set the resin layer while applying pressure to the outsides of the first and second substrates. According to the foregoing preferred structure, a resin layer satisfactorily free from irregularity in thickness can be formed in a short time.
It is preferable that the method of manufacturing an optical information recording medium according to the present invention further comprises an applying step of applying an adhesive-bonding layer, which is transparent with respect to the light beam, to the upper surface of either of the second substrates of a pair of optical information recording mediums obtained by the method of manufacturing an optical information recording medium according to the present invention; and an adhesive-bonding step of causing the second substrates of the pair of optical information recording mediums to face each other and adhesive-bonding the second substrates to each other through the resin layer. According to the foregoing, a recording medium having a four-layer structure can be obtained by repeating, three times, the adhesive-bonding step for obtaining the recording medium having the double-layer structure. That is, the recording medium having the four-layer structure can be manufactured by a method similar to that for manufacturing the recording medium having the double-layer structure by basically preparing the same manufacturing apparatuses for the three steps. In the foregoing case, it is preferable that the thicknesses of the first substrates of the pair of optical information recording mediums are substantially the same and the thicknesses of the second substrates of the pair of optical information recording mediums are substantially the same.
According to another aspect of the present invention, there is provided an apparatus for manufacturing an optical information recording medium comprising: a first substrate support section for supporting a first substrate; a second substrate support section disposed to face the first substrate support section so as to support a second substrate; an applying section for applying a resin layer to the upper surface of the first substrate or the second substrate; a spacer disposed on a plane on which the first substrate support section and the second substrate support section face each other, the spacer being disposed outward from the first and second substrates; and a pressing section for elevating the first substrate support section or the second substrate support section and pressing the first and second substrates. The apparatus for manufacturing an optical information recording medium allows, a recording medium having a multilayer structure to be produced efficiently, and it can be expected that the recording capacity can be increased.
It is preferable that the apparatus for manufacturing an optical information recording medium according to the present invention further comprises a light source disposed opposite to a surface of the first substrate support section that is in contact with the first substrate, wherein the first substrate support section is made of a material that permits penetration of a portion of light emitted from the light source. According to the foregoing preferred structure, the photosetting resin layer is used so that the first substrate and the second substrate are adhesive-bonded to each other in a short time.
It is preferable that the apparatus for manufacturing an optical information recording medium according to the present invention has a structure such that each of the first and second substrates has a central opening in the central portion thereof and concentric or spiral projection and pit columns or guide grooves on either surface thereof, and a center position correction section for making the central axes of the information pit columns or guide grooves of the first substrate and the second substrate to coincide with each other is further provided for at least one of the first substrate support section and the second substrate support section. According to the foregoing preferred structure, a recording medium can be obtained in which the deviation of the circular arc of the information pit columns or the guide grooves formed on the surfaces of the two information layers can be inhibited.
It is preferable that the apparatus for manufacturing an optical information recording medium according to the present invention further comprising first and second shaft sections respectively disposed on the central axes of the first and second substrate support sections; and first and second inner-portion guide sections each having a tapered portion having an end larger than the central opening of the substrate and another end smaller than the central opening of the substrate, the first and second inner-portion guide sections being capable of moving along the first and second shaft sections. According to the foregoing preferred structure, the first and second substrates can be secured to the surfaces of the first and second substrate support sections in the state where the tapered portions of the first and second inner-portion guide sections respectively are in contact with the central holes of the first and second substrates. Therefore, the central axis of the first information layer and that of the first substrate support section can be made coincide with each other with a value near the limit of mechanical accuracy. Moreover, the central axis of the second information layer and that of the second substrate support section can be made coincide with each other with a degree near the limit of mechanical accuracy. In the foregoing case, it is preferable that a projecting tapered section is formed on the leading end of one of the first shaft section or the second shaft section, and a corresponding recessed tapered portion is formed in the residual leading end of the other. According to the foregoing preferred structure, by moving the second substrate support section downwards, the projecting tapered portion formed on the leading end of either of the first shaft section or the second shaft section and the recessed tapered portion formed in the leading end of the other are engaged to each other. Thus, the central axis of the first information layer and that of the second information layer can be made to coincide with each other.
According to another aspect of the present invention, there is provided an apparatus for optically recording and reproducing information which irradiates, with light, an optical information recording medium having two different information layers provided with one or more selected from the group consisting of information pits corresponding to information signals, guide grooves for tracking and sample pits so as to be capable of reproducing information signals from the information layers, recording information signals on the information layers and reproducing the recorded information signals. The apparatus for optically recording and reproducing information comprises: optical means for causing an objective lens to converge a light beam emitted from a light source onto the recording medium; focus control means for performing control to make the focal point of the light beam coincide with either of the information layers; tracking control means for controlling the position of the light beam to enable the light beam to follow the information pits, guide grooves or the sample pits; layer identification means for demodulating a signal for identifying the information layers in accordance with light reflected by or transmitted through the information pits; layer selection means for selecting an information layer from which the information signal is reproduced or on which the same is recorded; and switch means for switching a method of tracking, which is performed by the tracking control means, to correspond to a result of selection performed by the layer selection means. According to the foregoing preferred structure of the an apparatus for optically recording and reproducing information, information can be recorded on and reproduced from the recording medium having a multilayer structure.
It is preferable that the apparatus for optically recording and reproducing information according to the present invention further comprises layer comparison means for subjecting, to a comparison a result of selection performed by the layer selection means and a result of identification performed by the layer identification means, and a focus jumping circuit for generating a pulse voltage to move the focal point of the light beam between the information layers to correspond to an output from the comparison means. According to the foregoing preferred structure, if an information layer, which is not the subject, is focused, the focusing position can be moved to the subject information layer. In the foregoing case, it is preferable further to provide tracking polarity inverting means for switching the polarity of the tracking control means in synchronization with the operation of the focus jumping circuit. According to the foregoing preferred structure, even in a case of a recording medium having the first information layer and the second information layer having information pits formed in the opposite directions between the two layers when viewed from a position upon which the light beam is made incident, the light beam can instantaneously be moved onto the information pit of the subject information layer.
It is preferable that the apparatus for optically recording and reproducing information according to the present invention has a structure such that the focus control means includes a first focus control means for performing control to make the focal point of the light beam to coincide with a position near the information layer, second focus control means permitted to be operated in a range smaller than that of the first focus control means, and focus switch means for performing switching to the second focus control means after the operation of the first focus control means has been completed. According to the foregoing preferred structure, servo operation of each information layer can be performed stably while maintaining the focus pulling performance similar to that obtainable from the conventional structure.
It is preferable that the apparatus for optically recording and reproducing information according to the present invention has a structure such that the focus control means has at least two types of operation conditions to be adaptable to each of the information layers, the focus control means being arranged to select one of the operation conditions in accordance with a result of selection performed by the layer selection means. According to the foregoing preferred structure, focus deviation can be corrected so that information can be recorded or reproduced satisfactorily.
It is preferable that the apparatus for optically recording and reproducing information according to the present invention has a structure such that the tracking control means has at least two types of operation conditions to be adaptable to each of the information layers, the tracking control means being arranged to select one of the operation conditions in accordance with a result of selection performed by the layer selection means. According to the foregoing preferred structure, tracking deviation can be corrected so that information can be recorded or reproduced satisfactorily.
It is preferable that the apparatus for optically recording and reproducing information according to the present invention further comprises a photodetector for performing a focus control including a first divisional light receiving surface for receiving a portion of light reflected by the recording medium and a second divisional light receiving surface for receiving the reflected light on the same plane on which the first divisional light receiving surface is positioned, the second divisional light receiving surface receiving the reflected light on the outside of the first divisional light receiving surface. According to the foregoing preferred structure, when information is reproduced from a recording medium having a multilayer structure, the range in which the focus is pulled can be enlarged by switching the focus detection region between the moment when the focus is pulled and the moment when the servo operation is performed.